Biological testing has become an important tool in detecting and monitoring diseases. In the biological field, thermal cycling is utilized in order to perform polymerase chain reactions (PCR) and other reactions. To amplify DNA (deoxyribonucleic acid) using the PCR process, a specifically constituted liquid reaction mixture is cycled through a PCR protocol including several different temperature incubation periods. An aspect of the PCR process is the concept of thermal cycling: alternating steps of melting DNA, annealing short primers to the resulting single strands, and extending those primers to make new copies of double-stranded DNA. During thermal cycling, it may be desirable that the temperature of each of a plurality of sample wells is substantially uniform. In addition, it may be desirable that condensation be avoided on the caps or other covering for the sample wells.
One method of inhibiting condensation on the top of the sample wells is to provide a heated platen for pressing down on the tops or caps of the sample well trays. Such a pressing force is often accomplished by using one or more springs located around a periphery of the platen. The platen may typically be included as part of a cover and may typically be metal. The platen may transfer heat to the caps of the sample wells by utilizing a resistive element connected to the platen, thereby inhibiting condensation. In addition, the platen may press down on the sample well(s) so that the sample well outer conical surfaces are pressed firmly against the mating surfaces on the sample block. This may increase heat transfer to the sample well(s) and facilitate a uniform distribution of sample well temperatures. The platen may also prevent thermal leakage from the interior of the device. Examples of a system with a heated platen are described in U.S. Pat. Nos. 5,475,610, 5,602,756, and 5,710,381, all of which are assigned to the assignee of the present invention, and the contents of which are all hereby incorporated by reference herein.
It may also be desirable for the heated platen to allow optical access to the sample wells. To achieve this, the heated platen may have a plurality of optical openings passing through the heated platen to allow light to pass from a light source to the samples and then to a detection device. These holes, while providing the benefit of allowing optical access, also may facilitate heat loss through the holes and/or radiated from the upper surface of the platen. In addition to heat loss, as the thermal cycling device is used over time, dust or other foreign particles may accumulate in the optical openings, thus reducing the amount of light that may transmit through the heated platen.